The invention relates to membranes comprising a blend of at least one thermoplastic polyolefin component having crystallinity derived from the presence of polypropylene sequences, an inorganic filler, and a processing oil.
Blends of one or more polyolefin polymers, one or more fillers, and one or more process oil plasticizers have historically been used to form membranes having a desired balance of properties for severe service conditions such as roof sheeting. Such composition are disclosed in U.S. Pat. Nos. 5,854,327, 5,859,114, 5,468,550, 5,582,890, 5,370,755, 5,389,715, 5,242,970, 5,286,798, and 5,256,228, among others. In general, the polymer compositions described in these inventions comprise ethylene-propylene-diene terpolymers (EPDM) or other rubber components that are cured or crosslinked to form a thermoset composition and/or materials high in ethylene crystallinity. Typically, any crystallinity present in these blends is derived from polyethylene sequences.
The term xe2x80x9cmembrane,xe2x80x9d as used herein, refers to films useful in a variety of uses such as roof sheeting, geo membranes, pond liners, and other applications requiring long term exposure to extreme environmental conditions with minimal degradation. These films are typically much thicker, possess less elasticity, and require greater long term resistance to severe environmental conditions resulting from roof movement, heavy winds, freeze-thaw cycles, and high temperatures than their lighter duty, disposable counterparts such as those films used in food and medical applications. It would be particularly advantageous for such membranes to be produced from thermoplastic compositions that possess adequate or superior tensile and tear strength properties from polypropylene crystallinity without the need for curing or crosslinking, thereby facilitating their production in conventional thermoplastic processing equipment. It would further be desirable for such membranes to possess a degree of tack sufficient to produce a desired level of bonding force where the edges of multiple sheets of the membrane material meet and overlap to form a continuous barrier, for example, over an entire rooftop.
For these membranes, it would be particularly advantageous for such thermoplastic membranes to have the following properties:
1. Tensile elongation, as measured by ASTM Method D412 greater than 100%, more preferably greater than 300% and most preferably greater than 500%.
2. Tensile strength greater than 5 Mpa, as measured by ASTM Method D412 preferably greater than 10 MPa and more preferably greater than 13 MPa.
3. Shore hardness, as measured by ASTM Method D412 less than 100 Shore A, preferably less than 80 Shore A and more preferably less than 60 Sh A
4. Softening point, greater than 45xc2x0 C., preferably greater than 60xc2x0 C. and most preferably greater than 80xc2x0 C.
5. A glass transition temperature substantially below 0xc2x0 C.
6. Small changes in these properties on exposure to environmental conditions for an extended period.
7. Self tack greater than 10 KN/m2 
U.S. Pat. No. 3,888,949 suggests the synthesis of blend compositions containing isotactic polypropylene and copolymers of propylene and an alpha-olefin, containing between 6-20 carbon atoms, which have improved elongation and tensile strength over either the copolymer or isotactic polypropylene. Copolymers of propylene and alpha-olefin are described wherein the alpha-olefin is hexene, octene or dodecene. However, the copolymer is made with a heterogeneous titanium catalyst resulting in copolymers with non-uniform composition distribution and a broad molecular weight distribution. Non-uniform intramolecular compositional distribution is evident in U.S. Pat. No. 3,888,949 by the use of the term xe2x80x9cblockxe2x80x9d in the description of the polymer where the copolymer is described as having xe2x80x9csequences of different alpha-olefin content.xe2x80x9d Within the context of the invention described above the term sequences describes a number of olefin monomer residues linked together by chemical formed during polymerization.
More recently several authors have shown the formation of more refined structures of partially atactic, partially isotactic polypropylene which have elastomeric properties. It is believed that in these components each molecule consists of portions which are isotactic and therefore crystallizable while the other portions of the same polypropylene molecule are atactic and therefore amorphous and not crystallizable. Examples of these propylene homopolymers containing different levels of isotacticity in different portions of the molecule are described in U.S. Pat. No. 5,594,080, in the article in the Journal American Chemical Society (1995), 117, p. 11586; in the article in the Journal American Chemical Society (1997), 119, p. 3635; in the journal article in the Journal of the American Chemical Society (1991), 113, pp. 8569-8570, and in the journal article in the Journal Macromolecules (1995) 28, pp. 3771-3778. These articles describe the copolymer of the present composition but do not describe the compositions obtained in blends with a more crystalline polymer such as isotactic polypropylene, nor its resultant desirable physical properties.
U.S. Pat. Nos. 3,853,969 and 3,378,606, suggest the formation of in situ blends of isotactic polypropylene and xe2x80x9cstereo blockxe2x80x9d copolymers of propylene and another olefin of 2 to 12 carbon atoms, including ethylene and hexene. The copolymers of this invention are necessarily heterogeneous in intermolecular and intramolecular composition distribution. This is demonstrated by the synthesis procedures of these copolymers which involve sequential injection of monomer mixtures of different compositions to synthesize polymeric portions of analogously different compositions. In addition, FIG. 1 of both patents shows that the xe2x80x9cstereo blockxe2x80x9d character, which is intra or intermolecular compositional differences in the context of the description of the present invention, is essential to the benefit of the tensile and elongation properties of the blend of these patents. Moreover, all of these compositions either do not meet all of the desired properties for various applications.
Similar results are purportedly achieved in U.S. Pat. No. 3,262,992 wherein the authors suggest that the addition of a stereoblock copolymer of ethylene and propylene to isotactic polypropylene leads to improved mechanical properties of the blend compared to isotactic polypropylene alone. However, these benefits are described only for the stereoblock copolymers of ethylene and propylene. These copolymers were synthesized by changing the monomer concentrations in the reactor with time. This is shown in examples 1 and 2. The stereoblock character of the polymer is graphically shown in the molecular description (column 2, line 65) and contrasted with the undesirable random copolymer (column 2, line 60). The presence of stereoblock character in these polymers is shown by the high melting point of these polymers and the poor solubility in hydrocarbons at ambient temperature.
The invention relates to a thermoplastic membrane (e.g. film, fluid barrier, and the like) which is formed from a blend of:
a) from 30 to 90 percent by weight of a first polymer component (FPC), wherein said FPC comprises a propylene-ethylene copolymer containing from 80 to 93 percent by weight of units derived from propylene, and having a melting point derived from stereoregular polypropylene sequences, as determined by differential scanning calorimetry (DSC), in the range of from 25xc2x0 C. to 70xc2x0 C., and a heat of fusion of from 2 J/g to 25 J/g;
b) from 1 to 40 percent by weight of an inorganic filler; and
c) from 1 to 25 percent by weight of a processing oil,
wherein the sum of components in said is 100 percent.
In one embodiment, the membrane further comprises up to 40 percent by weight of a second polymer component (SPC), wherein said SPC is a stereoregular polypropylene containing less than 7 percent by weight of units derived from ethylene or alpha-olefin monomer containing from 4 to 8 carbon atoms and having melting point, as determined by DSC, greater than 130xc2x0 C., and a heat of fusion greater than 80 J/g.
In another embodiment, the membrane of the invention contains an FPC which is a blend of two polymers. The additional first polymer component (FPC2) is a second propylene-ethylene copolymer containing at least 75 weight percent units derived from propylene and at least 5 weight percent less units derived from propylene than the first propylene-ethylene copolymer of the FPC. Preferably the FPC2 has a melting point less than 100xc2x0 C. and a heat of fusion less than 60 J/g.